Particulate mineral materials find use in a variety of different applications including, but not limited to, coatings, pigments, fillers, proppants, catalysts, extenders, inert carriers, for filtration, for insulations, and for horticultural applications. One example of a particulate mineral material is perlite. Perlite is a naturally occurring siliceous volcanic glass rock, generally distinguishable from other volcanic glasses due to its expansion from about four to about twenty times its original volume when heated to a temperature within its softening range. Perlite particles have been found to be useful in an array of applications, such as those mentioned above, both in their expanded and in their unexpanded form.
The expanded form of perlite may be achieved due in part to the presence of water trapped within the crude perlite glass rock. When perlite is quickly heated, the water vaporizes, creating bubbles in the heat-softened glassy particles and generally resulting in a light-weight, chemically inert, highly expanded perlite product. An expanded perlite product may be manufactured to weigh from, for example, about 2 pds/ft3 to about 15 pds/ft3, allowing it to be adapted for numerous uses, such as those previously described.
The final form and grade of an expanded perlite product may be controlled by, among other things, changing the heating cycle within a perlite expander, altering the size profile of an unexpanded perlite feed material by milling, or other processes now known to those of skill in the art or hereafter discovered. In one expanded form, the perlite particles are aggregate particles. In another expanded form, the perlite particles are solid microspheres. In a further expanded form, the perlite particles are porous microspheres. Expanded perlite in the form of porous microspheres has, in general, fewer inner cells compared to the relatively larger number of inner cells found in the more commonly produced expanded perlite aggregate particles.
Perlite may additionally be milled after it is expanded. Expanded perlite that has not been subsequently milled generally has a foamy or bubbly structure and may include porous spheres. When expanded perlite is subsequently milled, the bubbles in the structure are generally crushed, resulting in bubble fragments that are smaller and generally of a platy structure.
Another example of a particulate mineral material is diatomaceous earth (also called “DE” or “diatomite”), which is generally regarded as a sediment enriched in biogenic silica (i.e., silica produced or brought about by living organisms) in the form of siliceous skeletons (frustules) of diatoms. Diatoms are a diverse array of microscopic, single-celled, golden-brown algae of the class Bacillariophyceae that possess an ornate siliceous skeleton of varied and intricate structures comprising two valves that, in the living diatom, fit together much like a pill box. In one embodiment, the diatomaceous earth is freshwater diatomite. In another embodiment, the diatomaceous earth is saltwater diatomite.
In some applications, the compressive strength, hardness, and/or color of the particulate mineral materials may play an important roll in fulfilling their intended purpose. It has been known in the art to improve certain attributes of particulate mineral materials through surface coating or binding particles together. U.S. Pat. No. 3,849,149 to Swift et al., for example, appears to disclose a method of modifying the surface properties of certain particulate mineral materials with a surface coating having a significant number of acidic sites with pKa values less than 2.8, so as to result in greater ease of incorporation and uniformity of dispersion in pigment and filler end uses. As another example, U.S. Pat. No. 4,432,798 to Helferich et al. appears to disclose moldable, self-setting composition consisting of a granular or particulate aggregrate held together by an alkali-aluminosilicate binder hydrogel. As a further example, U.S. Pat. No. 5,352,287 to Wason et al. appears to disclose a composite pigment product that comprises a mineral nucleus coated with a substantially continuous and uniform active paper pigment coating, which may be used to enhance the opacity, brightness, and/or optical performance characteristics of paper. As another example, U.S. Pat. No. 6,641,908 B1 to Clough appears to disclose inorganic substrate materials comprising a metal oxide coating that may be formed by high temperature plasma coating.
The present inventors have unexpectedly discovered that, by coating particulate mineral materials with at least one metal compound, the compressive strength, hardness, and/or color of those materials may be improved. In one embodiment, the at least one metal compound is a metal oxide compound. In another embodiment, the at least one metal compound is a metal silicate compound. None of the references mentioned above appear to teach or suggest at least the following: (1) the use of at least one metal silicate coating to improve the compressive strength, hardness, and/or coloration of at least one particulate mineral material; (2) the application of at least one zirconium or zinc silicate coating onto at least one diatomite particulate mineral material; (3) the application of at least one aluminosilicate coating onto at least one perlite particulate mineral material; and (4) the coating of at least one particulate mineral material with a metal oxide coating, by use of low (e.g., room) temperature solution coating.